Changes in the induction and expression of synaptic plasticity
Because glutamatergic LTD has been scarcely explored in the PP – DG synapses, a relevant finding of this study was the establishment of a protocol to induce reliable LTD. The exploratory experiments designed to this end showed that electrical stimulation of MPP, but not LPP, induces a stable LTD in response to 900 pulses delivered at 3Hz, a stimulation frequency that mimics the delta–theta range of volley activity of stellar cells and pyramidal neurons of EC layer II synapsing DG granule cells (Gloveli et al., 1997; Deshmukh et al., 2010). In a previous study, Peñasco et al. demonstrated that MPP LTD can be induced with 6000 pulses at 10 Hz, but this required simultaneous blockade of GABAA receptors and involved activation of the CB1R (Peñasco et al., 2019). In the present study, LTD was induced with less electrical stimulation, and GABAergic transmission remained active; the resulting LTD was stable for up to 90 minutes without visible signs of a return to the baseline fEPSP value. The LTD was accompanied by increased PPR, indicating a presynaptic locus for its expression. As in Peñasco et al., the MPP LTD reported in our study requires postsynaptic production of 2-AG.
In slices from MK-801-treated animals, LFS induced synaptic potentiation instead of LTD. The absence of LTD may result from presynaptic dysregulation in CB1R activity, accelerated breakdown of 2-AG, or altered synthesis of 2-AG at the postsynaptic level. The latter is unlikely, since physostigmine induced a synaptic depression like that observed with activation of CB1R in the MK-801 condition. These findings indicate that MK-801-treated animals maintain functional postsynaptic production of 2-AG, suggesting a potential dysregulation locus at the presynaptic MPP terminals. In this regard, a critical finding of this study was that the blockade of the MAGL enzyme reverted LTD loss in MK-801-treated animals. MAGL is a presynaptically expressed enzyme that metabolizes 2-AG and is essential in multiple physiological processes, including neuroplasticity, cognitive performance, and behavior (Wang et al., 2018a; Zanfirescu et al., 2021). Our results suggest that transient hypofunction of NMDARs interferes with the expression of the MAGL enzyme, affecting the MPP’s ability to express long-term depression of glutamatergic transmission.
We also documented that transient hypofunction of NMDARs hinders the induction and expression of TBS-induced LTP in the LPP synapse and attenuates the magnitude but not the expression of LTP in the MPP synapse. While MPP LTP may be induced in MK-801-treated slices through a series of redundant signaling pathways, including CaMKII and PKA-ERK1/2 (Wu et al., 2006; Welsby et al., 2009), that assure its postsynaptic expression, in the LPP – DG synapse, LTP requires postsynaptic synthesis of 2-AG (Wang et al., 2016), presynaptic activation of CB1R, and increased neurotransmitter release mediated by the non-canonic CB1R /FAK/ROCK pathway (Wang et al., 2018b). Although we did not investigate the mechanisms underlying the blunted induction of LPP LTP, decreased functional expression of CB1R receptors or accelerated degradation of 2-AG may explain the absence of LTP observed in this study.